A VVT (Variable Valve Timing) apparatus has conventionally been known that changes the phase (crank angle) in (at) which an intake valve or an exhaust valve is opened/closed, according to an operating condition. Generally, in the variable valve timing apparatus, the phase is changed by rotating a camshaft, which opens/closes the intake valve or exhaust valve, relative to a sprocket or the like. The camshaft is rotated by an actuator such as a hydraulic or electric motor.
For accurate control of the valve timing using such a variable valve timing, it is necessary to detect the actual valve timing (that is, camshaft phase) at present, with high accuracy. In this regard, Patent Document 1 (PCT application National Publication No. 04-506851) discloses a variable valve timing apparatus in which camshaft phase is feedback-controlled through autocalibration of sensor measurement offset.
A hydraulic variable valve timing apparatus has a problem that control accuracy of variable valve timing deteriorates in cold climates or at the start of engine operation, as the hydraulic pressure decreases or response of hydraulic control slows down. Therefore, a variable valve timing apparatus using an electric motor as a driving source has been proposed (see, for example, Japanese Patent Laying-Open No. 2003-295953 and Japanese Patent Laying-Open No. 2004-162706).
Patent Document 2 (Japanese Patent Laying-Open No. 2003-295953) discloses, for a variable valve timing apparatus using a DC servomotor as an actuator, an arrangement preventing variation of response performance dependent on position of rotation of the controlled object and ensuring uniform control response at every controlled position of rotation. As to the detection of rotation angle (phase) of the controlled object, however, this reference simply describes that it is detected by using an output of a rotation angle sensor. Any technique for improving detection accuracy is not discussed.
Patent Document 3 (Japanese Patent Laying-Open No. 2004-162706) discloses a variable valve timing apparatus allowing variable valve timing by changing the rotation speed of the motor with respect to the rotation speed of the camshaft. According to Patent Document 3, in the variable valve timing apparatus as such, based on a crank angle signal output from a crank angle sensor and on a cam angle signal output from a cam angle sensor, an actual valve timing at the time when the cam angle signal was output is calculated. Further, the amount of change of valve timing from the actual valve timing when the cam angle signal was output is calculated based on the difference in rotation speed between the motor and the camshaft, and using the actual valve timing when the cam angle signal was output and the amount of change of the valve timing, the final, actual valve timing is calculated. Therefore, even at a timing in the interval of cam angle signal generation, which signal is output every time the camshaft rotates by a prescribed angle, the valve timing can be detected in an interpolating manner.
When the engine speed is high and the interval of cam angle signal generation becomes short, it is not much necessary to execute calculation of the amount of change in valve-timing in the interpolating manner as in Patent Document 3. On the contrary, when the engine speed is low, particularly when the engine is stopped or the engine is in a low rotation range where the engine speed is unstable, it is difficult to detect with high accuracy the actual valve timing based on the crank angle signal and the cam angle signal. Namely, the range of engine speed has an influence on the accuracy of detecting actual valve timing.
In the variable valve timing apparatus according to Patent Document 3, the amount of change in valve timing is continuously calculated based on the difference in rotation speed between the motor and the camshaft. Therefore, computational load for calculating the valve timing increases. This may lead to an increased cost, for example, for applying a processor capable of high-speed, large-volume processing. Patent Documents 1 and 2 are silent about the necessary consideration of computational load inherent to the detection of actual valve timing.